1. Field of the Invention
The present invention provides a low-cost resonance tag, which indicates the temperature history and which is a new application of a wireless tag, wherein the temperature history of a substance to be measured can easily be checked.
2. Description of the Related Art
Japanese Patent Laid-Open No. 2004-245607 discloses a temperature-history sensor having a plurality of temperature switches and LC resonance circuits, in which the value of capacitance element is switched in accordance with turning on or turning off of the temperature switch.
The temperature-history sensor described in Japanese Patent Laid-Open No. 2004-245607 can keep the temperature history stepwise and irreversibly. However, a commercially available temperature switch is used. Therefore, a temperature-history sensor suitable for selecting a set temperature more easily has been required.
Japanese Patent Laid-Open No. 2004-144683 describes a tag with a temperature sensor having a resonance circuit composed of a capacitor formed by using a material which has a dielectric constant varying in accordance with temperature changes.
However, regarding the tag with the temperature sensor described in Japanese Patent Laid-Open No. 2004-144683, the variation of the dielectric constant in accordance with the temperature change is reversible. Therefore, The history of sensed temperature change must be stored in nonvolatile memory included in the tag.